So-called heat lightning is a thunderstorm that is very tall and very far away. According to this article, sound from thunder will only travel about 15 miles, but if lightning is high enough you can see it 100-200 miles away. So this suggests to me that if you hear thunder, the storm is probably close enough you should go inside to be safe, but if you only see lightning and don’t hear thunder (assuming you are in a reasonably quiet location), it might be okay to stay outside.